It doesn't work quite right with macOS Sierra & Safari.
When I click on the "Click here to Upload a Photo..." link, I get this screen. That's fine.
But most of the time, I scroll to bottom of the left pane in Finder and choose 'Photos' to access my photo library. Using this method, I can't select multiple pictures. If I hold down the Command key to select multiple images, it only lets me upload the first one I choose.
Edit: Actually the 2nd picture I dragged into the same upload appears to be a broken link.
I can drag and drop multiple files into the window IF I know what directory the files are in and the filenames I want to upload. I saved these screen captures to the desktop and I tried it that way and it worked.
The thing is that most of the time I had no idea what directory the pictures are in or their filenames. It's just sort of the photo management paradigm of macOS (and earlier versions of OS X).